This is a technique for visualizing volumes, in which volume data, obtained by computed tomography (CT) or magnetic resonance imaging (MRI) for example, can be displayed in a display area. Volume rendering (VRT: volume rendering technique) or gradient-magnitude rendering are known examples of such visualization techniques. In these rendering techniques, optical properties have to be assigned to the 3D image data, present in the form of a matrix of scalar values. This is effected by a suitable choice of transfer function which takes into account the optical absorption and emission in the volume to be illustrated. This transfer function can also fix which parts of the volume to be displayed are illustrated opaquely, semi-transparently or transparently in the image. Moreover, in the case of color images, this transfer function assigns defined colors to the individual volume elements (voxels). When being displayed in the RGB color space, this is generally effected by the known value triplet (R, G, B).
In order to calculate the pixels to be displayed from the 3D image data, it is possible to use the so-called ray-casting algorithm, in which each pixel of the image is calculated by integrating or summing along a ray from the eye of the observer through the volume surrounded by the 3D image data. The corresponding transfer function is found in the sum or the integral, and thus the transparencies and colors of the individual voxels along the ray are summed using this.
In the process, the 3D image data is often rendered into an image with a bright image background. However, a large number of users prefer a dark image background, particularly if the image is intended to be looked at in a dark room. For this purpose, some applications allow the background illumination of the image to switched-off, so that the rendered 3D image data of the object now appears on a black image background. However, as a result of this, many images have a deteriorated image contrast, particularly in the field of medical imaging.
US 2007/0236496 A1 describes a graphic art display method for CT images, in which techniques such as duplication, symmetry inversion, contrast inversion, superposition of a number of images or deformation of the image contents, if need be combined with coloring, are applied to obtain a graphic art display. However, the document does not disclose a procedure for improved imaging on a dark image background.
US 2006/0007244 A1 discusses image processing which also uses a volume rendering technique and assigns colors to individual pixels by way of a color table. However, this document does not provide any indications for improved imaging on a dark image background either. The same applies to the article by E. K. Fishman et al.: “Volume Rendering versus Maximum Intensity Projection in CT Angiography: What Works Best, When and Why”, Radio Graphics, May-June 2006, vol. 26, no. 3, pages 905-923. This article compares different rendering techniques to one another without, however, discussing the problems associated with imaging on a dark image background.